A raw material comprising a polymer compound has characteristic gas permeability for each raw material. Based on properties thereof, a desired gas component can be separated by allowing selective permeation by means of a membrane constituted of a specific polymer compound. As an industrial application embodiment of this gas separation membrane, study has been conducted for separating and recovering carbon dioxide from a large-scale carbon dioxide source in a thermal power station, a cement plant, a blast furnace in a steel plant or the like in relation to a global warming issue. Then, this membrane separation technique attracts attention as a solution to an environmental issue to allow achievement by relatively small energy. Meanwhile, natural gas or bio gas (gases generated by fermentation and anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops, and the like) is mainly a mixed gas of methane and carbon dioxide. Study has been made so far for a membrane separation method as a means for removing an impurity such as carbon dioxide therein (see Patent Literature 1 and Patent Literature 2). Specifically, study has been made for cellulose or polyimide as a raw material in purification of a natural gas. However, the membrane is plasticized under high pressure conditions and high carbon dioxide concentration in an actual plant, and a decrease of separation selectivity due to the plasticization has become a problem (see Non-Patent Literature 1, p. 313-322; and Non-Patent Literatures 2 and 3). In order to suppress plasticization of the membrane, introduction of crosslinked structure into a polymer compound constituting the membrane is known to be effective, and research has been continued for improvement in a polyimide membrane (see Non-Patent Literature 1, p. 3-27). Specific examples of arts utilizing a membrane having a crosslinked structure for the gas separation membrane include arts described in Patent Literature 3, and Non-Patent Literatures 4, 5 and 6.